Distillate fuels such as diesel fuels tend to exhibit reduced flow at reduced temperatures due in part to formation of solids in the fuel. The solids, which are wax crystals, have a slightly higher density than the distillate fuels at a given temperature, and as a result there is a tendency for the wax to settle to the bottom of the storage container. The reduced flow of the distillate fuel affects the transport and use of the distillate fuels not only in the refinery but also in an internal combustion engine. If the distillate fuel is cooled to below a temperature at which solid formation begins to occur in the fuel, generally known as the cloud point (ASTM D 2500) or wax appearance point (ASTM D 3117), solids forming in the fuel in time will essentially prevent the flow of the fuel, plugging piping in the refinery, during transport of the fuel, and in inlet lines supplying an engine. Under low temperature conditions during consumption of the distillate fuel, as in a diesel engine, wax precipitation and gelation can cause the engine fuel filter to plug. Wax formation and settling can occur in the fuel tank after an extended period of non-use, such as overnight, and increase the chances of engine failure because of nonuniform wax enrichment. The same problem of wax settling can occur on a larger scale in fuel storage tanks. Under conditions where the fuel still flows after solids have formed in the fuel, an effect known as channeling may occur. When the outlet valve on the container is opened, the initial fuel flow will be wax enriched. Then, a channel is created in the wax layer, allowing a quantity of liquid fuel depleted in wax to flow. The low-wax fuel will continue to flow if the container is not refilled or agitated. The final portion of fuel flowing from the container will then be highly wax enriched.
As used herein, distillate fuels encompass a range of fuel types, typically including but not limited to kerosene, intermediate distillates, lower volatility distillate gas oils, and higher viscosity distillates. Grades encompassed by the term include Grades No. 1-D, 2-D and 4-D for diesel fuels as defined in ASTM D 975, incorporated herein by reference. The distillate fuels are useful in a range of applications, including use in automotive diesel engines and in non-automotive applications under both varying and relatively constant speed and load conditions.
The wax settling behavior of a distillate fuel such as diesel fuel is a function of its composition. The fuel is comprised of a mixture of hydrocarbons including normal paraffins, branched paraffins, olefins, aromatics and other non-polar and polar compounds. As the diesel fuel temperature decreases at the refinery, during transport, storage, or in a vehicle, one or more components of the fuel will tend to separate, or precipitate, as a wax.
The components of the diesel fuel having the lowest solubility tend to be the first to separate as solids from the fuel with decreasing temperature. Straight chain hydrocarbons, such as normal paraffins, typically have the lowest solubility in the diesel fuel. Generally, the paraffin crystals which separate from the diesel fuel appear as individual crystals. As more crystals form in the fuel, they tend to agglomerate and eventually reach a particle size which is too great to remain suspended in the fuel.
It is known to incorporate additives into diesel fuel to enhance the flow properties of the fuel at low temperatures. These additives are generally viewed as operating under either or both of two primary mechanisms. In the first, the additive molecules have a configuration which allows them to interact with the n-paraffin molecules at the growing ends of the paraffin crystals. The interacting additive molecules by steric effects act as a cap to prevent additional paraffin molecules from adding to the crystal, thereby limiting the dimensions of the existing crystal. The ability of the additive to limit the dimensions of the growing paraffin crystal is evaluated by low temperature optical microscopy or by the pour point depression (PPD) test, ASTM D 97, incorporated herein by reference.
In the second mechanism, the flow modifying additive may improve the flow properties of diesel fuel at low temperatures by functioning as a nucleator to promote the growth of smaller size crystals. This modified crystal shape enhances the flow of fuel through a filter, and the ability of the additive to improve flow by altering the n-paraffin crystallization behavior is normally evaluated by tests such as the Cold Filter Plugging Point (CFPP) Test, IP 309, incorporated herein by reference.
Additional, secondary, mechanisms involving the modification of wax properties in the fuel by incorporation of additives include, but are not limited to, dispersal of the wax in the fuel and solubilization of the wax in the fuel.
A number of additives may be incorporated into distillate fuels for various reasons to adjust various characteristics of the fuel, such as cloud point, pour point or cold filter plugging point. However, additives introduced to improve these characteristics may have an antagonistic effect on the wax anti-settling properties of the fuel. For example, incorporating a flow improving additive having a higher density constituent, such as vinyl acetate, will improve the flow characteristics of the fuel but will also increase the density of any wax crystals containing the additive. As will be discussed below, increasing the density of the wax crystal relative to the liquid fuel tends to undesirably accelerate the settling rate of the wax.
The wax crystals forming in a fuel normally have a slightly higher density than the liquid fuel portion. Consequently, when the fuel in a storage container cools to temperatures below the cloud point, crystals will form and will tend to settle to the bottom of the container. The rate of wax settling is dependent on the properties of the liquid fuel, primarily the density and viscosity, and the size and shape of the wax crystals. Stokes Law quantitatively describes the relationship, wherein the settling rate is a function of the solid crystal diameter, solid crystal density, liquid density and the fuel viscosity at a particular temperature, according to the following equation
##EQU1## where R = settling rate (cm/sec) D = diameter of crystal (cm) d.sub.c = crystal density (g/cm.sup.3) d.sub.L = liquid density (g/cm.sup.3) G = gravitational constant = 981 cm/sec.sup.2 V = fuel viscosity (poise)
At a temperature of -10.degree. C. where the difference in density between crystal and liquid is about 0.1 g/cm.sup.3 and the fuel viscosity is 10 cSt (0.08 poise), reducing the crystal particle size from 100 microns to 10 microns will reduce the settling rate from 0.25 meter/hr to 0.06 meter/day under static conditions.
The range of available diesel fuels includes Grade No. 2-D, defined in ASTM D 975-90 (incorporated herein by reference) as a general purpose, middle distillate fuel for automotive diesel engines, which is also suitable for use in non-automotive applications, especially in conditions of frequently varying speed and load. Certain of these Grade No. 2-D (No. 2) fuels may be classified as being hard to treat when using one or more additives to improve flow. A hard-to-treat diesel fuel is either unresponsive to a flow improving additive, or requires increased levels of one or more additives relative to a normal fuel to effect flow improvement.
Fuels in general, and diesel fuels in particular, are mixtures of hydrocarbons of different chemical types (i.e., paraffins, aromatics, olefins, etc.) wherein each type may be present in a range of molecular weights and carbon lengths. The tendency of suspended solid waxes to settle is a function of one or more properties of the fuel, the properties being attributed to the composition of the fuel. For example, in the case of a hard-to-treat fuel the compositional properties which render a fuel hard to treat relative to normal fuels include a narrower wax distribution; the virtual absence of very high molecular weight waxes, or inordinately large amounts of very high molecular weight waxes; a higher total percentage of wax; and a higher average normal paraffin carbon number range. It is difficult to generate a single set of quantitative parameters which define a hard-to-treat fuel. Nevertheless, measured parameters which tend to identify a hard-to-treat middle distillate fuel include a temperature range of less than 100.degree. C. between the 20% distilled and 90% distilled temperatures (as determined by test method ASTM D 86 incorporated herein by reference), a temperature range less than 25.degree. C. between the 90% distilled temperature and the final boiling point (see ASTM D 86), and a final boiling point above or below the temperature range 360.degree. to 380 C.
Hard-to-treat fuels are particularly susceptible to wax settling phenomena due to the composition of the fuel. In a hard-to-treat fuel a large quantity of wax tends to settle at a faster rate. Fuel enhanced in long chain wax components tend to exhibit faster separation of wax crystals. Also, fuels with a narrow wax distribution tend to exhibit more sudden precipitation of wax crystals.
The phenomenon of wax settling out of a fuel manifests itself in static environments, such as during bulk storage or in a fuel tank. Where sufficient wax separates from and settles out of the fuel mixture, engine flow is effectively impeded or even interrupted completely. There continues to be a demand for additives which improve the wax anti-settling characteristics of distillate fuels. Further, there remains a need for additive compositions which are capable of improving the wax anti-settling properties of hard-to-treat fuels.